The present invention relates to apparatus for measuring the viscosity of fluids by determining pressure differentials.
The measurement of viscosity is fundamental to a multitude of industrial processes and many types of apparatus have been developed. The Saybolt viscometer measures the time rate of flow for a fixed volume of fluid to flow through a fine capillary tube. The viscosity is linearly proportional to the drain time according to Equation (1).                     μ        =                              (                                          π                ⁢                                  xe2x80x83                                ⁢                                  D                  4                                ⁢                                  h                  L                                ⁢                γ                                            128                ⁢                                  V                  L                                ⁢                L                                      )                    ⁢          t                                    (        1        )            
where
D is the tube diameter
hL is the fluid head loss
xcex3 is the fluid specific weight
VL is fluid volume exiting the tube in time t
L is the tube length
t is time
Another method measures the time for a given mass, usually a sphere, to fall through a fixed dimension, usually stagnant, column of fluid. Viscosity is linearly proportional to the measured fall time according to Equation (2).
for D/DT less than ⅓                    μ        =                              [                                                            D                  2                                ⁡                                  (                                                            γ                      S                                        -                    γ                                    )                                                            18                ⁢                                  L                  ⁡                                      (                                          1                      +                                                                        9                          ⁢                          D                                                                          4                          ⁢                                                      D                            T                                                                                              +                                                                        (                                                                                    9                              ⁢                              D                                                                                      4                              ⁢                                                              D                                T                                                                                                              )                                                2                                                              )                                                                        ]                    ⁢          t                                    (        2        )            
where
D is the sphere diameter
xcex3S is the sphere specific weight
xcex3 is the fluid specific weight
DT is the tube diameter
t is time
A variation of this method measures the time for a bubble to rise through a column of fluid.
A third method, known as the Brookfield viscometer measures the force or torque to overcome viscous shear forces between fixed and moving surfaces submerged in a fluid. These surfaces usually take the form of concentric cylinders. With this method, viscosity is linearly proportional to the measured force or torque according to Equation (3).                     μ        =                              F            ⁢                          xe2x80x83                        ⁢            S                                A            ⁢                          xe2x80x83                        ⁢            V                                              (        3        )            
where
F is the measured force
S is the fluid thickness
A is the wetted area
V is the differential velocity
The aforementioned methods of measuring viscosity are not well adapted to continuous measurement of process fluids as they require batch processing of fixed fluid volumes or involve stagnant columns of fluid. In response, another class of viscometer using a flow-through geometry was introduced where the pressure drop through a friction tube is measured when a liquid is pumped through it at a constant flow rate, Q. From the Poiseuille law shown in Equation (4), the viscosity, xcexc is seen to be linearly proportional to the pressure differential, Pd.                     μ        =                              π            ⁢                          xe2x80x83                        ⁢                          P              d                        ⁢            γ            ⁢                          xe2x80x83                        ⁢                          D              4                                            128            ⁢            Q            ⁢                          xe2x80x83                        ⁢            L                                              (        4        )            
Stagnant or trapped fluid volumes increase the measurement error in a continuous system where the bulk fluid viscosity is to be measured and controlled. The trapped fluid properties are not representative of changing bulk fluid properties because they do not change at the same rate as the bulk fluid in response to a control signal. In general, the bulk fluid properties slowly diffuse into the trapped fluid volume, presenting a significant time lag between a control signal and the response.
It is an object of the present invention to provide an apparatus, which can effectively measure viscosity and does not use the linearity techniques employed by the prior art.
This object is achieved in an apparatus for measuring the viscosity of a Newtonian fluid, comprising:
(a) a tubular housing which defines an internal cavity;
(b) a fluid restriction structure disposed in a first portion of the cavity;
(c) a pressure measuring device disposed to measure the differential pressure in the cavity of the fluid which has flowed through the fluid restriction structure; and
(d) a device for applying pressure to the fluid upstream from the fluid restriction structure to cause the fluid to flow through the fluid restriction structure so that the pressure measuring device measures the pressure differential which is inversely proportional to the fluid viscosity squared.
It has been determined that viscosity measuring apparatus can make use of a continuous flow design where fluid under constant pressure flows through the apparatus at a flow velocity that is inversely proportional to the fluid viscosity. The differential pressure measured across upstream and downstream pointing Pitot tubes in the viscometer is proportional to the square of the flow velocity and is therefore inversely proportional to the fluid viscosity squared.
An important feature of the present invention is that the accuracy of viscosity measurements can be significantly improved. Moreover, measurements can be made using the same apparatus more frequently than in the prior art and in fact continuously, without adversely affecting the measurements.